Organometallic compounds include a number of highly toxic manufactured chemicals including organoarsenic, and organotin compounds used as pesticides or herbicides, as well as nickel tetracarbonyl and tetraethyllead produced as by-products of the petroleum industry. Trace elements such as platinum, mercury, cadmium, and lead used in the plating industry are often processed to organometallic form before being jettisoned in industrial waste streams.
Alkyl leads, especially tetraethyllead (TEL), represent a typical example of organometals described above. Handling practices at some manufacturing facilities have resulted in soil and ground water contamination by TEL and other alkyl leads. These compounds are highly toxic to the environment and detrimental to human health. TEL causes serious human health effects at doses of less than 10 mg/kg body weight and many TEL decomposition products (e.g., triethyl lead chloride) are also known to be toxic to humans. Many decomposition products are also quite water soluble making it possible for them to be carried long distances in ground water. Because many of the inorganic lead salts resulting from complete decomposition of TEL are relatively insoluble in water, inorganic lead (elemental symbol=Pb) has been identified as one of the more desirable decomposition products with regard to reducing soluble lead levels in the contaminated ground water. Organometals such as TEL generally show higher environmental mobility and/or greater toxicity than inorganic forms. It is environmentally beneficial to transform these organometals into their inorganic forms in order to prevent contamination of water supplies. Biological enzymatic activity is one such route to these transformations.
A number of methods have been described to remediate soil and ground water containing toxic chemicals. Colaruotolo et al. (U.S. Pat. No. 4,511,657) claim the use of specially adapted microbial cultures to treat obnoxious waste, especially halogenated organic chemical waste (U.S. Pat. No. 4,493,895). Methods for treating soil contaminated with toxic chemicals have also been described. Rehm and Kirchner (U.S. Pat. No. 4,871,673) claim the use of specially adapted microorganisms affixed to a porous, adsorptive carrier for the decontamination of soil. Chakrabarty and Kellogg (U.S. Pat. No. 4,535,061) claim the use of Pseudomonas cepacia ATCC 39027 and a mixed culture of Arthrobacter and Pseudomonas ATCC 39028 for the dissimulation of environmentally persistent chemical compounds in both soil and water. Peterson (U.S. Pat. No. 4,447,541) claims the use of a two part reagent mixture for hydrolyzing polyhalogenated organic compounds with subsequent biological removal of the hydrolysis products.
The use of bioreactors and in-situ stimulation of indigenous microflora are two current approaches to the decontamination of soil and ground water. Bioreactors have been designed to utilize microorganisms for the bioremediation of a variety of toxic contaminants, including trichloroethylene, phenol, and toluene. (Folsom et al., 1991, Applied and Environmental Microbiology, 57:1602-1608). In-situ bioremediation involves the growth of indigenous, contaminant-degrading microorganisms which are enhanced by adding nutrients and oxygen. Raymond (U.S. Pat. No. 3,846,290 and U.S. Pat. No. 4,588,506) claims a process in which oxygen and nutrients are supplied to biota for stimulating the biooxidation of hydrocarbons contaminating ground water without the addition of microorganisms to the contaminated environment. Other methods of in-situ bioremediation have targeted halogenated organic compounds such as trichloroethylene, vinyl chloride, and chloroform (Thomas et al., 1989, Environ. Sci. Technol., 23:760-766) and aromatic and polyaromatic hydrocarbons such as naphthalene and phenathrene. (Madsen et al., 1991, Science, 2:830-833).
Revis et al. (U.S. Pat. No. 4,826,602) claim that contacting aqueous waste with a culture of Pseudomonas maltophilica ATCC 53510 will reduce the concentration of ionic species of heavy metals. Macaskie (Macaskie et al., 1987, Environ. Technol. Lett., 8:635-640) has reported that alkyllead tolerant yeast strains derived from Candida humicola are able to degrade trimethyllead to inorganic lead. However, documented degradation of tetramethyllead in soil is not previously known in the art.
The methods cited above are useful and clearly show that microorganisms can be used to remove toxic compounds, from both soil and aqueous environments. There are however, several disadvantages to the methods outlined in the existing art. Examples given in the art describe decontamination of the environment using specific naturally occurring, or genetically engineered cultures of bacteria or yeast or the preliminary harsh chemical treatment of toxic contaminants prior to biological treatment by indigenous microbes. It should be noted that chemical treatment of the contaminated area is likely to kill the majority of the indigenous microbial flora and thus impede any subsequent remediation effort. Furthermore, the isolation or engineering, culturing, and inoculation of specific microorganisms particularly selected for the degradation of specific organic contaminants is labor intensive and time consuming. Bioreactors can allow for effective microbial growth with greater control over nutrient addition, temperature, pH, and concentration; however, in bioremediation projects, materials must be pumped out or excavated, and soils must be handled and sorted which is also labor intensive. Bioremediation efforts that utilize in-situ methods have been effective in degrading certain toxic compounds; however, they have not addressed the specific problem of organometal contamination.
It has not been clear in the prior art how to stimulate such transformations in natural microbial populations. Toxicity of the organometal contaminants to microbes is a potential problem in any scheme for biological transformation of organometals, (Macaskie et al., 1985, Environmental Technology Letters, 6:237-250) and stimulation of indigenous microbial populations naturally adapted to the toxic organometals provides a distinct advantage over the art. Furthermore, in the few attempts at demonstrating microbial organolead transformation in pure cultures, reports generally show such activity to be barely detectable (e.g., Macaskie and Dean, 1987, Environmental Technology Letters, 8:635-640). The methods proposed here of utilizing microbial flora indigenous to the contaminated area to effect biotransformation of toxic organolead compounds eliminate the need for isolation or engineering of specific microorganisms and clearly demonstrate a statistically significant increase in the level of transformation of organolead compounds. Thus, the present invention provides a process whereby toxic compounds, and specifically organolead such as TEL, are converted to inorganic species via stimulation of the indigenous microbial population of the contaminated area.